In PoE, limited power is transmitted to Ethernet-connected equipment (e.g., VoIP telephones, WLAN transmitters, security cameras, etc.) from an Ethernet switch. In one type of PoE system, called PoDL, DC power from the switch is transmitted over a single twisted wire pair. The same twisted wire pair also transmits/receives differential data signals. In this way, the need for providing any external power source for the PDs can be eliminated. The standards for PoE and PoDL are set out in IEEE 802.3 and are well-known.
FIG. 1 illustrates conventional coupling/decoupling networks between a PSE 10 and a PD 12 in an Ethernet PoDL system. The PSE 10 is represented as a DC voltage source, but also may include a differential data transmitter or transceiver. The differential data may also be generated by any other circuit. The PD 12 is represented by a load that receives the DC voltage, and the PD 12 may include any circuitry that receives the differential data over the wire pair 14.
In the example of FIG. 1, DC power is delivered from the PSE 10 to the PD 12 through a single twisted wire pair 14 via a coupling network that conducts DC (or low frequency current), for power, between a DC voltage source and the wire pair 14, while simultaneously blocking the differential AC data (or high frequency current) from the DC voltage source. Similarly, the PD 12 uses a decoupling network that decouples the transmitted DC voltage for powering a PD load, while conducting only the PHYs' AC data to data terminals in the PD 12. The ability of the coupling/decoupling networks to block the PHYs' AC data over a very broad range of frequencies is a key requirement for PoDL Ethernet applications where the data rates may vary from less than 10Mbps to greater than 1Gbps. In the example of FIG. 1, the capacitors C1-C4 are intended to block DC in the data path, while the inductors L1-L4 are intended to block AC in the power path.
In FIG. 1, inductors L1-L4 are used to couple/decouple the DC flowing between the PSE 10 voltage source and the PD 12 load to/from the wires 14. The inductors L1-L4 are AC blocking devices whose impedance is proportional to frequency. The constant of proportionality is referred to as the inductance L. Because of parasitic capacitance between the two terminals of each inductor, the impedance may reach a maximum at a self-resonant frequency and then begin to decrease as frequency increases. The self-resonant frequency (SRF) can be expressed as SRF =1/{2π√(LCparasitic)} where Cparasitic is the parasitic intra-winding capacitance of the inductor.
The ability of a single inductor to impede AC over a broad range of frequencies depends on the magnitude of inductance, the inductor's ability to conduct DC current without losing its inductance, and its parasitic capacitance. Because of the broadband nature of the digital data being transmitted between the PHYs of the PSE and PD, it may not be possible for a single inductor to maintain enough impedance in shunt with the PHYs' terminations over the required bandwidth, resulting in insufficient return loss at the wire pair connector.
The problem of passing DC while blocking AC in the power path over a broad bandwidth has been addressed with RF diplexers and bias tees, but these devices are designed for unbalanced coaxial transmission lines and hence are unsuitable for PoDL applications, which rely upon unshielded, balanced, twisted pair data lines. Such unsuitable broadband bias tees make use of cascaded inductors with the requisite snubbing, thus overcoming the limitations of a single inductor's SRF in order to deliver sufficient broadband shunt impedance.
FIG. 2 illustrates a 4 stage broadband bias tee 20 found in contemporary literature, which is unsuitable for PoDL using unshielded twisted wire pair.
For PoDL applications, a fully balanced topology is required.
Thus, what is needed in the field of PoDL is an improved coupling/decoupling network that combines or separates the DC power and wide bandwidth AC data with suitably large return loss.